For two bands or carriers separated in frequency by a sufficient amount such that their in-band intermodulation distortion (“IMD”) does not overlap means that a high sample rate or a large bandwidth is used to provide a cavity filter response enveloping both bands as well as both in-band IMD terms. In the past, systems may employ envelope tracking (“ET”) by a single power supply for feeding a drain current to a power amplifier to improve efficiency in such power amplifier for a single band. However, providing a single power supply with sufficient bandwidth to cover a large cavity filter response adds a substantial amount of cost. For example, bandwidth for drain modulation, such as for supplying a drain current to a power amplifier, generally is at least twice the instantaneous bandwidth (“IBW”) of a signal. Because of bandwidth demands in more recent communication systems, such demand has in some instances precluded use of envelope tracking in power provided to a power amplifier.
Others have used wide band switching power supplies to provide a drain current to a power amplifier for envelope tracking. Again, as bandwidth for such switching power supplies is at least twice total bandwidth of a signal, for instances with widely spaced carriers a high degree of complexity and cost is added to provide such switching power supplies. Furthermore, efficiency of such switching power supplies may be depressed for configurations with wide separation of two or more carriers. Thus, system efficiency is affected by efficiency of having a single switching power supply to provide envelope tracking, as well as reduced efficiency of a power amplifier provided with less efficient or closeness of envelope tracking by a drain current associated with widely separated carriers.
Accordingly, it would be useful and desirable to provide envelope tracking which overcomes one or more of the above-described limitations.